1. Field of the Invention
The present invention relates in general to a nanocomposite. More particularly, it relates to an epoxy/clay nanocomposite and its use as matrix material for printed circuit boards.
2. Description of the Related Arts
Circuit boards find a wide variety of uses in the electrical industry such as radios, televisions, and various electrical apparatus. A widely employed technique for preparing circuit boards is to impregnate a woven fiberglass sheet with a resin composition and then laminate a copper sheet to one or both sides of the resin impregnated fiberglass sheet. Next, an electrical circuit is etched into the copper to form the circuit board and then electrical connections can be soldered to the board when it is used.
Epoxy resins have been widely used for the purpose of impregnating the fiberglass to prepare the circuit boards. However, the circuit boards thus prepared are not satisfactory when a higher degree of heat resistance, dimensional stability, or less hygroscopicity is required. Therefore, an improvement upon the epoxy resin for use as the matrix material for printed circuit boards is called for. To this end, the present invention proposes using an epoxy/clay nanocomposite as the matrix material for printed circuit boards.
Nanocomposites are a new class of materials that exhibit ultra-fine phase dimensions, typically in the range of 1-100 nm. Experimental work on these materials has generally shown that virtually all types and classes of nanocomposites lead to new and improved properties such as increased stiffness, strength, and heat resistance, and decreased moisture absorption, flammability, and permeability, when compared to their micro- and macrocomposite counterparts. Specifically, commercially available Nylon 6/clay nanocomposite shows that polymer matrix having layered clay minerals dispersed therein exhibits improved mechanical strength, heat distortion temperature (HDT), and impermeability to gas and water.
An object of the invention is to provide an-epoxy/clay nanocomposite for use as the matrix material for printed circuit boards which can lead to improved thermal and dimensional stability and reduced hygroscopic property.
Another object of the invention is to provide a prepreg for printed circuit boards which contains the epoxy/clay nanocomposite as the matrix material.
A further object of the invention is to provide a printed circuit board made by at least one of the above prepregs.
To achieve the above objects, a layered clay material is modified by ion exchange with (1) benzalkonium chloride and (2) a hardener of dicyandiamide or tetraethylenepentamine. The modifiers used herein can funcitonalize the clay layers and expand the interlayer spacing thereof. The modified clay material is then blended with epoxy oligomers to undergo polymerization. The silicate layers of the clay material are exfoliated during the polymerization and uniformly dispersed throughout the epoxy resin matrix on a nanometer length scale. Thereby, an epoxy/clay nanocomposite suitable as the matrix material for printed circuit boards is obtained with reduced hygroscopicity and improved thermal and dimensional stability.
The epoxy/clay nanocomposite in accordance with the present invention includes a polymer matrix comprising an epoxy resin; and a layered clay material uniformly dispersed in the polymer matrix, wherein the layered clay material has been modified by ion exchange with (1) benzalkonium chloride and (2) dicyandiamide or tetraethylenepentamine.
The layered clay material used in the present invention is preferably a layered silicate having a cation-exchange capacity ranging from about 50 to 200 meq/100 g. The layered silicate suitable for use herein includes, for example, smectite clay, vermiculite, halloysite, sericite, mica, and the like. Illustrative of suitable smectite clays are montmorillonite, saponite, beidellite, nontronite, hectorite, and stevensite.
The layered silicate is subjected to intercalation of two distinct modifiers by ion exchange to thereby functionalize the clay material and expand the interlayer spacing between the adjacent silicate layers, such that the silicate layers are more readily exfoliated during the composite formation. The ion-exchange operation can be accomplished by immersing the layered silicate in an aqueous solution containing the modifier, followed by washing the treated layered silicate with water to remove-excess ions. The first modifier used in the present invention is benzalkonium chloride, which serves to introduce a hydrophobic group to improve the compatibility between the clay and the epoxy resin. Thus, a better uniformity can be achieved when the clay layers are exfoliated and dispersed throughout the epoxy matrix. The second modifier is a hardener of either dicyandiamide or tetraethylenepentamine. The hardener can afford a reactive functional group to the clay material to promote bonding with the epoxy resin. By this, the thermal and dimensional stability or other properties can be improved to a large extent even when a small amount of clay is incorporated.
The epoxy/clay nanocomposite of the present invention is prepared by dispersing the above-mentioned modified clay material in oligomers of an epoxy resin, and polymerizing the oligomers into an epoxy polymer. In accordance with the present invention, the modified clay material is preferably present in an amount ranging from about 0.1% to 10% by weight, and more preferably from about 0.5% to 6.0% by weight, based on the total weight of the epoxy/clay composite. It is preferable that the clay material contained in the polymer matrix has interlayer spacing of at least about 18 xc3x85. The epoxy resin suitable for use in the present invention includes but is not limited to bisphenol A type epoxy resins, brominated epoxy resins (bromine content: 5-60 wt %), novolac epoxy resins, multifunctional epoxy resins, and aliphatic epoxy resins. A mixture of the above is also suitable for use. Exemplary epoxy resins include bisphenol A epoxy resin, tetrabromo bisphenol A epoxy resin, tetrabromo bisphenol A polyphenol epoxy resin, ortho-cresol novolac epoxy resin, N,N,Nxe2x80x2,Nxe2x80x2-tetra(2,3-epoxypropyl)-Pxe2x80x2,Pxe2x80x2-methylaniline, N,N-bis(2,3-epoxypropyl)-4-amino-phenylepoxypropyl ether, 4-epoxypropylene-N,N-bisepxoypropylaniline and the like.
The epoxy/clay nanocomposite of the present invention may further comprise an ordinary epoxy curing agent such as dicyandiamide, phenol novolak, or trimellitic anhydride (TMA). The amount of the curing agent to be used is 0.7 to 1.2 equivalents based on the epoxy group. An amount of the curing agent of lower than 0.7 equivalents or over 1.2 equivalents based on the epoxy group may result in insufficient curing. In addition, the epoxy/clay nanocomposite may further comprise a curing accelerator commonly used for accelerating the curing of an epoxy resin. The curing accelerator includes, for example, imidazole compounds such as 2-ethyl-4-methylimidazole and 1-benzyl-2-methylimidazole; and tertiary amines such as Nxe2x80x2,N-dimethylbenzylamine (BDMA). These compounds can be used singly or in a form of mixture. The curing accelerator should be used in a small amount as far as the accelerator is sufficient for accelerating the curing of the epoxy resin. The amount of the curing accelerator to be used is preferably between 0.1 and 1 parts by weight based on 100 parts by weight of the epoxy resin.
The epoxy/clay nanocomposite of the invention is preferably employed to prepare printed circuit boards. In preparing the boards, a fibrous substrate is coated and impregnated with a varnish containing the composite of the present invention. Suitable organic solvents for preparing the varnish include N,N-dimethylformamide, acetone, isopropanol, ethylene glycol monomethyl ether, propylene glycol monomethyl ether, butanol, and methyl ethyl ketone. Subsequent to coating, the impregnated substrate is dried and partially cured to form a dry substrate called a prepreg.
The prepreg thus obtained can be used for manufacturing a copper-clad laminate, a multi-layered laminate, or a printed circuit board by conventional methods well known in the art. For example, a sheet of copper or other conductive materials can be laminated to one or more layers of the prepreg. Then a circuit can be etched to the conductive layer using techniques well-known to form circuit boards. The laminates prepared in accordance with the present invention possess a high dimensional and thermal stability and a low water uptake. In the preferred embodiments of the invention, the laminates can have a coefficient of thermal expansion (CTE) of less than 60 ppm/xc2x0 C., more preferably less than 50 ppm/xc2x0 C. before the glass transition temperature (Tg) in the thickness (Z) direction; the water uptake can be less than 0.5 wt %, more preferably less than 0.42 wt % under the conditions of 2 hours and 120xc2x0 C. in a pressure cooker; the durable time in a 288xc2x0 C. solder bath can be greater than 3 minutes.